Current-mode amplifier

ABSTRACT

A current-mode amplifier including an input stage, a feedback circuit and an output stage is provided. The input stage has an input terminal for receiving an input current of the current-mode amplifier. The input stage generates a corresponding inner current in accordance with the input current and a feedback current. The feedback circuit is connected to the input stage. The feedback circuit generates the corresponding feedback current in accordance with the inner current of the input stage. An input terminal of the output stage is connected to an output terminal of the input stage. An output terminal of the output stage serves as an output terminal of the current-mode amplifier.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the priority benefit of Taiwan applicationserial no. 99123838, filed on Jul. 20, 2010. The entirety of theabove-mentioned patent application is hereby incorporated by referenceherein and made a part of specification.

BACKGROUND

1. Field of the Disclosure

The disclosure relates to an amplifier. More particularly, thedisclosure relates to a current-mode amplifier.

2. Description of Related Art

In an ultra-wideband (UWB) wireless transceiver system, signal data istransmitted by orthogonal frequency division multiplexing (OFDM). Todecode the signal data at a transceiver end, during a process ofreducing signal from a radio frequency (RF) to a baseband (0-240 MHz),and an operation of an analog to digital converter (ADC), a gain of thebaseband must be consistent. However, in a present circuit design, avoltage-mode amplifier cannot be operated over 100 MHz. In a receiver ofthe UWB system, since the circuit is usually operated in a non-linearzone due to interference signals during a process of reducing the signalfrom the RF to the baseband, a linearity consideration is veryimportant. In the UWB system, a variable gain amplifier must have a goodlinearity, an optimal direct current (DC) offset and an acceptableanti-noise function.

In a conventional IF down-conversion design, design difficulties andpower consumptions of the variable gain amplifier and a filter areincreased as a bandwidth thereof is increased. Also the IFdown-conversion can resolve the problem of DC offset, other problems areencountered, for example, linearity and power consumption. Therefore, adirection-converter is still widely used in the UWB system. Anyway, inthe present UWB system, the voltage-mode amplifier is used to implementthe variable gain amplifier. An input impedance of the voltage-modeamplifier is the greater the better (which preferably approachesinfinity), and an output impedance thereof is the smaller the better(which preferably approaches 0). Contrary to the voltage-mode amplifier,an input impedance of a current-mode amplifier is the smaller the better(which preferably approaches 0), and an output impedance thereof is thegreater the better (which preferably approaches infinity). In thepresent UWB system, the current-mode amplifier is not yet used toimplement the variable gain amplifier.

SUMMARY

The disclosure is directed to a current-mode amplifier, which can beused to implement a variable gain amplifier of an ultra-wideband (UWB)system.

An exemplary embodiment of the disclosure provides a current-modeamplifier including an input stage, a feedback circuit and an outputstage. The input stage has an input terminal for receiving an inputcurrent of the current-mode amplifier. The input stage generates acorresponding inner current according to the input current and afeedback current of the feedback circuit. The feedback circuit isconnected to the input stage. The feedback circuit generates thecorresponding feedback current according to the inner current of theinput stage. An input terminal of the output stage is connected to anoutput terminal of the input stage. An output terminal of the outputstage serves as an output terminal of the current-mode amplifier.

According to the above descriptions, an exemplary embodiment of thedisclosure provides a current-mode amplifier, which can be used toimplement a variable gain amplifier in a UWB wireless transceiversystem. A baseband signal is amplified according to a broadband propertyof the current-mode amplifier, so as to overcome a limitation that thevoltage-mode amplifier cannot be operated over 100 MHz. In thedisclosure, a gain control/change of the current-mode amplifier isachieved by controlling a current mirror, so that a problem of gainerror influenced by fabrication variation can be resolved.

In order to make the aforementioned and other features and advantages ofthe disclosure comprehensible, several exemplary embodiments accompaniedwith figures are described in detail below.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings are included to provide a furtherunderstanding of the disclosure, and are incorporated in and constitutea part of this specification. The drawings illustrate embodiments of thedisclosure and, together with the description, serve to explain theprinciples of the disclosure.

FIG. 1 is a functional block diagram illustrating a current-modeamplifier according to an exemplary embodiment of the disclosure.

FIG. 2 is a circuit schematic diagram of a current-mode amplifier ofFIG. 1 according to an exemplary embodiment of the disclosure.

FIG. 3 is a circuit schematic diagram illustrating a common-modefeedback amplifier according to an exemplary embodiment of thedisclosure.

FIG. 4 is a circuit schematic diagram illustrating a current-modeamplifier of FIG. 1 according to another exemplary embodiment of thedisclosure.

FIG. 5 is a diagram illustrating a simulation result of an inputimpedance of a feedback circuit of FIG. 4.

FIG. 6 is a gain frequency response diagram of a current-mode amplifierof FIG. 4.

FIG. 7 is a circuit schematic diagram illustrating a current-modeamplifier 100 of FIG. 1 according to still another exemplary embodimentof the disclosure.

FIG. 8 is a circuit schematic diagram illustrating a common-modefeedback amplifier according to another exemplary embodiment of thedisclosure.

FIG. 9 is a circuit schematic diagram illustrating a current-modeamplifier of FIG. 1 according to yet another exemplary embodiment of thedisclosure.

DETAILED DESCRIPTION OF DISCLOSED EMBODIMENTS

An amplifier fabricated according to a complementary metal-oxidesemiconductor (CMOS) process has a high voltage gain. Regarding theamplifier with a fixed current consumption, multiplication of a gain Gand a bandwidth W thereof is a constant C (i.e. G×W=C). Therefore, toobtain a relatively high bandwidth W and relatively high gain G, powerconsumption has to be increased. However, a baseband requirement ofultra-wideband (UWB) cannot be reached regardless of how greater thepower consumption is, so that a method of adding a zero pole to theamplifier is used to increase the multiplication of the gain and thebandwidth. However, when the bandwidth becomes greater (greater than 250MHz), such method is inapplicable.

A conventional analog baseband circuit structure generally uses avoltage-mode transmission approach without using a current-modetransmission approach. Regarding a broadband system, the smaller a loadresistance is, the wider an operation bandwidth is, and the smaller theload resistance is, the better effect the current transmission is. Suchcharacteristic avails applying a current-mode amplifier to a present ora future UWB system.

FIG. 1 is a functional block diagram illustrating a current-modeamplifier 100 according to an exemplary embodiment of the disclosure. Asshown in FIG. 1, the current-mode amplifier 100 includes a feedbackcircuit 110, an input stage 120 and an output stage 130. The input stage120 has an input terminal for receiving an input current I_(in) of thecurrent-mode amplifier 100. The input stage 120 generates acorresponding inner current according to the input current I_(in) and afeedback current of the feedback circuit 110. The input stage 120 mayuse a common gate complementary metal-oxide semiconductor (CMOS)amplifier in collaboration with the feedback circuit 110 to achieve aneffect of reducing an input impedance. In other exemplary embodiment,the input stage 120 may use a common base bipolar junction transistor(BJT) amplifier in collaboration with the feedback circuit 110 toachieve the effect of reducing the input impedance.

The feedback circuit 110 is connected to the input stage 120. Thefeedback circuit 110 generates the corresponding feedback currentaccording to the inner current of the input stage 120. In some exemplaryembodiments, the feedback circuit 110 can be a trans-impedance amplifier(TIA). A relationship of the inner current, the input current I_(in) andthe feedback current is determined according to a design requirement.For example, the inner circuit is a sum of the input current I_(in) andthe feedback current. The feedback circuit 110 may reduce the inputimpedance of the input stage 120, so as to achieve a purpose ofbroadband.

An input terminal of the output stage 130 is connected to an outputterminal of the input stage 120. An output terminal of the output stage130 serves as an output terminal of the current-mode amplifier 100. Theoutput stage 130 generates a corresponding output current I_(out)according to the inner current of the input stage 120. The output stage130 has one or a plurality of gain circuits, for example, K gaincircuits 130-1, 130-2, . . . , 130-K shown in FIG. 1. According to again requirement, each of the gain circuits 130-1˜130-K can enable apart of the gain circuits by controlling a bias voltage thereof, anddisable the other gain circuits. By enabling and disabling the gaincircuits 130-1˜130-K, a gain of the current-mode amplifier 100 isadjusted, so as to achieve a variable gain effect.

FIG. 2 is a circuit schematic diagram of the current-mode amplifier 100of FIG. 1 according to an exemplary embodiment of the disclosure. In thepresent exemplary embodiment, the input current I_(in) includes a firstinput current I_(in+) and a second input current I_(in−), and the outputcurrent I_(out) includes a first output current I_(out+) and a secondoutput current I_(out−). In FIG. 2, two gain circuits 130-1 and 130-2connected in parallel are used to implement the output stage 130, thougha number of the gain circuits can be determined according to an actualdesign requirement. For example, the gain circuit 130-2 can be omitted,or more gain circuit 130-2 can be applied.

The input terminal of the input stage 120 includes a first inputterminal used for receiving the first input current I_(in+) and a secondinput terminal used for receiving the second input current I_(in−). Theoutput terminal of the input stage 120 include a first output terminalused for providing a first inner signal 101 and a second output terminalused for providing a second inner signal 102. Based on a current mirrorstructure 210, the input stage 120 may convert a first inner currentI_(inner+) and a second inner current I_(inner−) into the correspondingfirst inner signal 101 and the second inner signal 102. The output stage130 can mirror the inner currents I_(inner+) and I_(inner−) of the inputstage 120 to internal of the output stage 130 according to the firstinner signal 101 and the second inner signal 102.

The input stage 120 includes a first current source 121, a secondcurrent source 122, a first transistor M1, a second transistor M1 b, athird transistor M2 and a fourth transistor M2 b. A first end of thefirst current source 121 is connected to a first end (for example, asource) of the first transistor M1, a second end of the first currentsource 121 is connected to a first reference voltage (for example, apower voltage VDD). The first end of the first transistor M1 is furtherconnected to the first input terminal of the input stage 120, so thatthe first end of the first transistor M1 may receive a current Iss ofthe first current source 121 and the first input current I_(in+) of theinput stage 120. A first end of the second current source 122 isconnected to a first end (for example, a source) of the secondtransistor M1 b, and a second end of the second current source 122 isconnected to the first reference voltage. The first end of the secondtransistor M1 b is further connected to the second input terminal of theinput stage 120, so that the first end of the second transistor M1 b mayreceive the current Iss of the second current source 122 and the secondinput current I_(in−) of the input stage 120.

Control ends (for example, gates) of the first transistor M1 and thesecond transistor M1 b are controlled by the feedback circuit 110. Asecond end (for example, a drain) of the first transistor M1 isconnected to a first end (for example, a drain) of the third transistorM2. A control end (for example, a gate) of the third transistor M2 isconnected to the first end of the third transistor M2. A common node ofthe third transistor M2 and the first transistor M1 is connected to thefirst output terminal of the input stage 120 for providing the firstinner signal 101. A second end (for example, a drain) of the secondtransistor M1 b is connected to a first end (for example, a drain) ofthe fourth transistor M2 b. A control end (for example, a gate) of thefourth transistor M2 b is connected to the first end of the fourthtransistor M2 b. A common node of the fourth transistor M2 b and thesecond transistor M1 b is connected to the second output terminal of theinput stage 120 for providing the second inner signal 102. Second ends(for example, sources) of the third transistor M2 and the fourthtransistor M2 b are connected to a second reference voltage (forexample, a ground voltage).

In the present exemplary embodiment, the first transistor M1 and thesecond transistor M1 b are P-channel metal oxide semiconductor (PMOS)transistors, and the third transistor M2 and the fourth transistor M2 bare N-channel metal oxide semiconductor (NMOS) transistors.

The feedback circuit 110 includes a first impedance 111, a secondimpedance 112, a fifth transistor M3 and a sixth transistor M3 b. Thefirst impedance 111 and the second impedance 112 can be impedancedevices such as resistors, transistors, or diodes, etc. In the presentexemplary embodiment, the first impedance 111 includes a seventhtransistor M4, and the second impedance 112 includes an eighthtransistor M4 b. In the present exemplary embodiment, the fifthtransistor M3, the sixth transistor M3B, the seventh transistor M4 andthe eighth transistor M4 b are NMOS transistors.

A first end (for example, a drain) of the seventh transistor M4 isconnected to the first reference voltage, and a second end (for example,a source) of the seventh transistor M4 is connected to the control endof the first transistor M1. A first end (for example, a drain) of thefifth transistor M3 is connected to the second end of the seventhtransistor M4, a second end (for example, a source) of the fifthtransistor M3 is connected to the second reference voltage, and acontrol end (for example, a gate) of the fifth transistor M3 isconnected to the first end of the third transistor M2. The thirdtransistor M2 and the fifth transistor M3 form a current mirror, so thatthe feedback circuit 110 can generate a corresponding feedback currentI_(fb+) according to the inner current I_(inner+) of the input stage120. The seventh transistor M4 may convert the feedback current I_(fb+)into a corresponding control voltage to control the first transistor M1,so as to reduce the input impedance of the input stage 120 and achievethe purpose of broadband. By adjusting a bias VB_(n1), a gain of thefeedback current I_(fb+) can be changed. In the present exemplaryembodiment, a relationship of the inner current I_(inner+), the inputcurrent I_(in+) and the feedback current I_(fb+) isI_(inner+)=I_(in+)+I_(fb+).

A first end (for example, a drain) of the eighth transistor M4 b isconnected to the first reference voltage, and a second end (for example,a source) of the eighth transistor M4 b is connected to the control endof the second transistor M1 b. A first end (for example, a drain) of thesixth transistor M3 b is connected to the second end of the eighthtransistor M4 b, a second end (for example, a source) of the sixthtransistor M3 b is connected to the second reference voltage, and acontrol end (for example, a gate) of the sixth transistor M4 b isconnected to the first end of the fourth transistor M2 b. The fourthtransistor M2 b and the sixth transistor M3 b form a current mirror, sothat the feedback circuit 110 can generate a corresponding feedbackcurrent I_(fb−) according to the inner current I_(inner−) of the inputstage 120. The eighth transistor M4 b may convert the feedback currentI_(fb−) into a corresponding control voltage to control the secondtransistor M1 b, so as to reduce the input impedance of the input stage120 and achieve the purpose of broadband. By adjusting the bias VB_(n1),a gain of the feedback current I_(fb−) can be changed. In the presentexemplary embodiment, a relationship of the inner current I_(inner−),the input current I_(in−) and the feedback current I_(fb−) isI_(inner−)=I_(in−)+I_(fb−).

The input terminal of the output stage 130 includes a first inputterminal used for receiving the first inner signal 101 and a secondinput terminal used for receiving the second inner signal 102, and theoutput terminal of the output stage 130 includes a first output terminalused for providing the first output current I_(out+) and a second outputterminal used for providing the second output current I_(out−). The gaincircuit 130-1 of the output stage 130 includes a ninth transistor M9, atenth transistor M10, an eleventh transistor M11 and a twelfthtransistor M12. In the present exemplary embodiment, the ninthtransistor M9 and the tenth transistor M10 are PMOS transistors, and theeleventh transistor M11 and the twelfth transistor M12 are NMOStransistors. A first end (for example, a drain) of the ninth transistorM9 is connected to the first output terminal of the output stage 130,and a control end (for example, a gate) of the ninth transistor M9receives a first bias VB. A first end (for example, a drain) of thetenth transistor M10 is connected to the second output terminal of theoutput stage 130, and a control end (for example, a gate) of the tenthtransistor M10 receives the first bias VB. Second ends (for example,sources) of the transistors M9 and M10 are connected to the firstreference voltage (for example, the power voltage VDD).

A first end (for example, a drain) of the eleventh transistor M11 isconnected to the first end of the ninth transistor M9, and a control end(for example, a gate) of the eleventh transistor M11 is connected to thefirst input terminal of the output stage 130 for receiving the firstinner signal 101. A first end (for example, a drain) of the twelfthtransistor M12 is connected to the first end of the tenth transistorM10, and a control end (for example, a gate) of the twelfth transistorM12 is connected to the second input terminal of the output stage 130for receiving the second inner signal 102. Second ends (for example,sources) of the transistors M11 and M12 are connected to the secondreference voltage (for example, the ground voltage).

The gain circuit 130-2 of the output stage 130 includes a first switchSWp, a thirteenth transistor M13, a fourteenth transistor M14, afifteenth transistor M15 and a sixteenth transistor M16. In the presentexemplary embodiment, the thirteenth transistor M13 and the fourteenthtransistor M14 are PMOS transistors, and the fifteenth transistor M15and the sixteenth transistor M16 are NMOS transistors. A first end (forexample, a drain) of the thirteenth transistor M13 is connected to thefirst output terminal of the output stage 130. A first end (for example,a drain) of the fourteenth transistor M14 is connected to the secondoutput terminal of the output stage 130. Second ends (for example,sources) of the transistors M13 and M14 are connected to the firstreference voltage (for example, the power voltage VDD).

A first end (for example, a drain) of the fifteenth transistor M15 isconnected to the first end of the thirteenth transistor M13, and acontrol end (for example, a gate) of the fifteenth transistor M15 isconnected to the first input terminal of the output stage 130 forreceiving the first inner signal 101. A first end (for example, a drain)of the sixteenth transistor M16 is connected to the first end of thefourteenth transistor M14, and a control end (for example, a gate) ofthe sixteenth transistor M16 is connected to the second input terminalof the output stage 130 for receiving the second inner signal 102.Second ends (for example, sources) of the transistors M15 and M16 areconnected to the second reference voltage (for example, the groundvoltage).

The first bias VB or the first reference voltage (for example, the powervoltage VDD) can be selected and transmitted to the control ends (thegates) of the thirteenth transistor M13 and the fourteenth transistorM14 through the first switch SWp. When the first bias VB is selected andtransmitted to the control ends of the transistors M13 and M14 throughthe first switch SWp, a circuit structure of the gain circuit 130-2 issimilar to that of the gain circuit 130-1. The transistors M2, M2 b,M11, M12, M15 and M16 form the current mirror structure 210. Thetransistors M2, M11 and M16 form a current mirror, wherein channelaspect ratios (or channel width/length ratios W/L) of the transistorsM2, M11 and M15 are 1:M:N. According to the first inner signal 101, thegain circuits 130-1 and the 130-2 can respectively mirror the innercurrent I_(inner+) to internals of the gain circuits 130-1 and 130-2 bymultiplication factors of M and N. The transistors M2 b, M12 and M16form another current mirror, wherein channel aspect ratios (or channelwidth/length ratios W/L) of the transistors M2 b, M12 and M16 are 1:M:N.According to the second inner signal 102, the gain circuits 130-1 andthe 130-2 can respectively mirror the inner current I_(inner−) tointernals of the gain circuits 130-1 and 130-2 by multiplication factorsof M and N. By determining the proportional relation of 1:M:N, currentgains of the gain circuits 130-1 and 130-2 can be set. Now, the gaincircuits 130-1 and 130-2 commonly provide the first output currentI_(out+) and the second output current I_(out−).

When the power voltage VDD is selected and transmitted to the controlends of the transistors M13 and M14 through the first switch SWp, thetransistors M13 and M14 are turned off, which is equivalent to asituation that the gain circuit 130-2 is disabled. Now, the first outputcurrent I_(out+) and the second output current I_(out−) are provided bythe gain circuit 130-1 alone. Therefore, by controlling the first switchSWp, the gain of the current-mode amplifier 100 can be changed, so as toachieve an effect of a variable gain amplifier (VGA).

The first bias VB is determined according to an actual designrequirement. For example, the first bias VB can be a band-gap referencevoltage or a common-mode feedback (CMFB) voltage. FIG. 3 is a circuitschematic diagram illustrating a common-mode feedback amplifier 300according to an exemplary embodiment of the disclosure. The common-modefeedback amplifier 300 includes a PMOS transistor M21, a PMOS transistorM22, an NMOS transistor M23, an NMOS transistor M24, an NMOS transistorM25 and a capacitor C1. Sources of the transistors M21 and M22 areconnected to the power voltage VDD. Gates of the transistors M21 and M22are connected to a drain of the transistor M22, a drain of thetransistor M24 and a first end of the capacitor C1. A drain of thetransistor M21 is connected to a drain of the transistor M23 and asecond end of the capacitor C1, and outputs the first bias VB to theoutput stage 130. A drain of the transistor M25 is connected to sourcesof the transistors M23 and M24, and a source of the transistor M25 isconnected to the ground. Gates of the transistors M23 and M24respectively receive a common-mode voltage and a feedback voltage of thesystem. A gate of the transistor M25 is connected to a bias VB_(n3). Thebias VB_(n3) can be a band-gap reference voltage or other fixedvoltages.

In the above exemplary embodiment, a broadband gain variablecurrent-mode amplifier structure is introduced. Such structure can bewidely applied to various wireless/cable broadband systems, and can beused to implement an analog baseband circuit in the broadband system foradjusting strength of a received signal.

FIG. 4 is a circuit schematic diagram illustrating a current-modeamplifier according to another exemplary embodiment of the disclosure.The current-mode amplifier 100 of FIG. 4 is similar to the current-modeamplifier 100 of FIG. 2, and a difference there between is that theoutput stage 130 of FIG. 4 further includes a second switch SWn, aseventeenth transistor M17, an eighteenth transistor M18, a nineteenthtransistor M19 and a twentieth transistor M20. In the present exemplaryembodiment, the transistors M17, M18, M19 and M20 are NMOS transistors.

The first end of the ninth transistor M9 is connected to the firstoutput terminal of the output stage 130 and a first end (for example, adrain) of the seventeenth transistor M17. The first end of the tenthtransistor M10 is connected to the second output terminal of the outputstage 130 and a first end (for example, a drain) of the eighteenthtransistor M18. A second end (for example, a source) of the seventeenthtransistor M17 is coupled to the first end of the eleventh transistorM11. A second end (for example, a source) of the eighteenth transistorM18 is connected to the first end of the twelfth transistor M12. Controlends (for example, gates) of the transistors M17 and M18 receive thesecond bias VB_(n2). The second bias VB_(n2) can be a band-gap referencevoltage or other fixed voltages.

The first end of the thirteenth transistor M13 is connected to the firstoutput terminal of the output stage 130 and a first end (for example, adrain) of the nineteenth transistor M19. The first end of the fourteenthtransistor M14 is connected to the second output terminal of the outputstage 130 and a first end (for example, a drain) of the twentiethtransistor M20. A second end (for example, a source) of the nineteenthtransistor M19 is coupled to the first end of the fifteenth transistorM15. A second end (for example, a source) of the twentieth transistorM20 is connected to the first end of the sixteenth transistor M16. Thesecond bias VB_(n2) or the second reference voltage (for example, theground voltage) is selected and transmitted to the control ends (thegates) of the transistors M19 and M20 through the second switch SWn.

When the first bias VB is selected and transmitted to the control endsof the transistors M13 and M14 through the first switch SWp, and thesecond bias VB_(n2) is selected and transmitted to the control ends (thegates) of the transistors M19 and M20 through the second switch SWn, thecircuit structure of the gain circuit 130-2 is similar to that of thegain circuit 130-1. Now, the gain circuits 130-1 and 130-2 commonlyprovide the first output current I_(out+) and the second output currentI_(out−), namely, the current-mode amplifier 100 has a relatively greatoutput gain.

When the power voltage VDD is selected and transmitted to the controlends of the transistors M13 and M14 through the first switch SWp, andthe ground voltage is selected and transmitted to the control ends ofthe transistors M19 and M20 through the second switch SWn, thetransistors M13, M14, M19 and M20 are turned off, so as to ensure thatthe disabled gain circuit 130-2 does not influence the output currentsI_(out+) and I_(out−) of the current-mode amplifier 100. Now, the firstoutput current I_(out+) and the second output current I_(out−) areprovided by the gain circuit 130-1 alone, namely, the current-modeamplifier 100 has a relatively small output gain.

In summary, based on a local feedback mode of the feedback circuit 110,a low impedance input stage is implemented by the transistors M1, M2, M1b and M2 b. The inner currents I_(inner+) and I_(inner−) of the inputstage 120 are respectively transmitted (mirrored) to a plurality of gaincircuits of the output stage 130 by a predetermined multiplicationfactor by using the current structure 210. By enabling/disabling thegain circuits, a tunable current gain stage of the output stage 130 isimplemented. An input impedance Z_(in) of the current-mode amplifier 100of FIG. 4 in an s-domain can be represented as a following equation (1):

$\begin{matrix}{Z_{in} = {\frac{1}{g_{m\; 1}} \cdot \frac{\begin{matrix}{{s^{2}C_{1}C_{2}} + {s\left( {{C_{1}g_{m\; 2}} + {C_{2}g_{m\; 4}}} \right)} +} \\\left( {{g_{m\; 4} \cdot g_{m\; 2}} - {g_{m\; 1} \cdot g_{m\; 3}}} \right)\end{matrix}}{\begin{matrix}{{s^{3}\frac{C_{1} \cdot C_{2} \cdot C_{in}}{g_{m\; 1}}} + {s^{2}\left\lbrack {{C_{1}C_{2}} + {\frac{C_{in}}{g_{m\; 1}}\left( {{C_{1}g_{m\; 2}} + {C_{2}g_{m\; 4}}} \right)}} \right\rbrack} +} \\{{s\left\lbrack {{C_{1}g_{m\; 2}} + {C_{2}g_{m\; 4}} + {\frac{C_{in}}{g_{m\; 1}}\left( {{g_{m\; 4}g_{m\; 2}} - {g_{m\; 1}g_{m\; 3}}} \right)}} \right\rbrack} + {g_{m\; 4} \cdot g_{m\; 2}}}\end{matrix}}}} & {{equation}\mspace{14mu} (1)}\end{matrix}$

Wherein, g_(m1), g_(m2), g_(m3) and g_(m4) respectively representconductances of the transistors M1, M2, M3 and M4. C_(in) represents aparasitic capacitance of the first input terminal of the input stage120. A parasitic capacitance C₁=C_(gs4)+C_(ds4)+C_(ds3), wherein C_(gs4)represents a parasitic capacitance from the gate to the source of thetransistor M4, C_(ds4) represents a parasitic capacitance from the drainto the source of the transistor M4, and C_(ds3) represents a parasiticcapacitance from the drain to the source of the transistor M3. Aparasitic capacitance C₂=C_(gs2)+C_(ds2)+C_(gs3), wherein C_(gs2)represents a parasitic capacitance from the gate to the source of thetransistor M2, C_(ds2) represents a parasitic capacitance from the drainto the source of the transistor M2, and C_(gs3) represents a parasiticcapacitance from the gate to the source of the transistor M3. In case ofa low frequency, the equation (1) can be simplified as:

$\begin{matrix}{Z_{{in},{DC}} = {\frac{1}{g_{m\; 1}} \cdot \left( {1 - {\frac{g_{m\; 1}}{g_{m\; 4}} \cdot \frac{g_{m\; 3}}{g_{m\; 2}}}} \right)}} & {{equation}\mspace{14mu} (2)}\end{matrix}$

Therefore, the input impedance can be reduced through the feedbackcircuit 110, so as to achieve the purpose of broadband.

FIG. 5 is a diagram illustrating a simulation result of the inputimpedance of the feedback circuit 110 of FIG. 4. According to FIG. 5, itis known that an input bandwidth of the feedback circuit 110 may reach 1GHz. FIG. 6 is a gain frequency response diagram of the current-modeamplifier 100 of FIG. 4. A vertical axis of FIG. 6 represents gains,i.e. ratios of the output current I_(out) and the input current I_(in).According to FIG. 6, it is known that under different gain modulations,the current-mode amplifier 100 may still maintain the bandwidth of 1GHz.

FIG. 7 is a circuit schematic diagram illustrating the current-modeamplifier 100 of FIG. 1 according to still another exemplary embodimentof the disclosure. The current-mode amplifier 100 of FIG. 7 is similarto the current-mode amplifier 100 of FIG. 2, and a difference therebetween is that the transistors M2, M2 b, M3, M3 b, M11, M12, M15 andM16 of FIG. 7 are PMOS transistors, and the transistors M1, M1 b, M4, M4b, M9, M10, M13 and M14 of FIG. 7 are NMOS transistors. In the presentembodiment, the first reference voltage connected to the transistors M1,M1 b, M3, M4 b, M9, M10, M13, M14 and the switch SWp is the groundvoltage, and the second reference voltage connected to the transistorsM2, M2 b, M3, M3 b, M11, M12, M15 and M16 is the power voltage VDD.

FIG. 8 is a circuit schematic diagram illustrating a common-modefeedback amplifier 800 according to another exemplary embodiment of thedisclosure. The common-mode feedback amplifier 800 of FIG. 8 is similarto the common-mode feedback amplifier 300 of FIG. 3, and a differencethere between is that transistors M23, M24 and M25 of the common-modefeedback amplifier 800 are PMOS transistors, and transistors M21 and M22of the common-mode feedback amplifier 800 are NMOS transistors. In thepresent exemplary embodiment, the first reference voltage connected tothe transistors M21 and M22 is the ground voltage, and the secondreference voltage connected to the transistor M25 is the power voltageVDD. The common-mode feedback amplifier 800 may generate the first biasVB to the output stage 130 of FIG. 7.

FIG. 9 is a circuit schematic diagram illustrating the current-modeamplifier 100 of FIG. 1 according to yet another exemplary embodiment ofthe disclosure. The current-mode amplifier 100 of FIG. 9 is similar tothe current-mode amplifiers 100 of FIG. 4 and FIG. 7.

In summary, the disclosure provides a variable gain amplifier, which canbe applied to a UWB wireless transceiver system. Signals within abaseband range (0-250 MHz) are amplified according to a broadbandproperty of the current-mode amplifier 100, so as to overcome alimitation that a voltage-mode amplifier cannot be operated over 100MHz. Moreover, a gain control of the current-mode amplifier 100 isachieved by controlling a current mirror, so that a problem of a gainerror influenced by fabrication variation can be resolved. An applicablebandwidth of the current-mode amplifier 100 can be more than 1 GHz,which can be very competitive in a future ultra-wideband application.

It will be apparent to those skilled in the art that variousmodifications and variations can be made to the structure of thedisclosure without departing from the scope or spirit of the disclosure.In view of the foregoing, it is intended that the disclosure covermodifications and variations of this disclosure provided they fallwithin the scope of the following claims and their equivalents.

1. A current-mode amplifier, comprising: an input stage, having an inputterminal for receiving an input current of the current-mode amplifier,wherein the input stage generates an inner current according to theinput current and a feedback current; an output stage, having an inputterminal connected to an output terminal of the input stage, and anoutput terminal serving as an output terminal of the current-modeamplifier; and a feedback circuit, having an input and an outputterminals connected to the input stage, for generating the feedbackcurrent according to the inner current of the input stage.
 2. Thecurrent-mode amplifier as claimed in claim 1, wherein the inner currentis a sum of the input current and the feedback current.
 3. Thecurrent-mode amplifier as claimed in claim 1, wherein the input terminalof the input stage comprises a first input terminal and a second inputterminal, the output terminal of the input stage comprises a firstoutput terminal and a second output terminal, and the input stagecomprises: a first current source; a second current source; a firsttransistor, having a first end connected to a first end of the firstcurrent source and the first input terminal of the input stage, and acontrol end being controlled by the feedback circuit; a secondtransistor, having a first end connected to a first end of the secondcurrent source and the second input terminal of the input stage, and acontrol end being controlled by the feedback circuit; a thirdtransistor, having a first end connected to a second end of the firsttransistor, and a control end connected to the first end of the thirdtransistor, wherein a common node of the third transistor and the firsttransistor is connected to the first output terminal of the input stage;and a fourth transistor, having a first end connected to a second end ofthe second transistor, and a control end connected to the first end ofthe fourth transistor, wherein a common node of the fourth transistorand the second transistor is connected to the second output terminal ofthe input stage.
 4. The current-mode amplifier as claimed in claim 3,wherein the first transistor and the second transistor are P-channelmetal oxide semiconductor (PMOS) transistors, and the third transistorand the fourth transistor are N-channel metal oxide semiconductor (NMOS)transistors.
 5. The current-mode amplifier as claimed in claim 3,wherein the first transistor and the second transistor are NMOStransistors, and the third transistor and the fourth transistor are PMOStransistors.
 6. The current-mode amplifier as claimed in claim 3,wherein second ends of the first current source and the second currentsource are connected to a first reference voltage, and second ends ofthe third transistor and the fourth transistor are connected to a secondreference voltage.
 7. The current-mode amplifier as claimed in claim 3,wherein the feedback circuit comprises: a first impedance, having afirst end connected to a first reference voltage, and a second endconnected to the control end of the first transistor; a secondimpedance, having a first end connected to the first reference voltage,and a second end connected to the control end of the second transistor;a fifth transistor, having a first end connected to the second end ofthe first impedance, a second end connected to a second referencevoltage, and a control end connected to the first end of the thirdtransistor; and a sixth transistor, having a first end connected to thesecond end of the second impedance, a second end connected to the secondreference voltage, and a control end connected to the first end of thefourth transistor.
 8. The current-mode amplifier as claimed in claim 7,wherein the first impedance comprises a seventh transistor, and thesecond impedance comprises an eighth transistor.
 9. The current-modeamplifier as claimed in claim 8, wherein the fifth, the sixth, theseventh and the eighth transistors are NMOS transistors.
 10. Thecurrent-mode amplifier as claimed in claim 8, wherein the fifth, thesixth, the seventh and the eighth transistors are PMOS transistors. 11.The current-mode amplifier as claimed in claim 1, wherein the inputterminal of the output stage comprises a first input terminal and asecond input terminal, the output terminal of the output stage comprisesa first output terminal and a second output terminal, and the outputstage comprises: a ninth transistor, having a first end connected to thefirst output terminal of the output stage, and a control end receiving afirst bias; a tenth transistor, having a first end connected to thesecond output terminal of the output stage, and a control end receivingthe first bias; an eleventh transistor, having a first end connected tothe first end of the ninth transistor, and a control end connected tothe first input terminal of the output stage; and a twelfth transistor,having a first end connected to the first end of the tenth transistor,and a control end connected to the second input terminal of the outputstage.
 12. The current-mode amplifier as claimed in claim 11, whereinthe output stage further comprises: a thirteenth transistor, having afirst end connected to the first output terminal of the output stage; afourteenth transistor, having a first end connected to the second outputterminal of the output stage; a first switch, selecting to transmit thefirst bias or a first reference voltage to control ends of thethirteenth transistor and the fourteenth transistor; a fifteenthtransistor, having a first end connected to the first end of thethirteenth transistor, and a control end connected to the first inputterminal of the output stage; and a sixteenth transistor, having a firstend connected to the first end of the fourteenth transistor, and acontrol end connected to the second input terminal of the output stage.13. The current-mode amplifier as claimed in claim 12, wherein theninth, the tenth, the thirteenth and the fourteenth transistors are PMOStransistors, and the eleventh, the twelfth, the fifteenth and thesixteenth transistors are NMOS transistors.
 14. The current-modeamplifier as claimed in claim 12, wherein the ninth, the tenth, thethirteenth and the fourteenth transistors are NMOS transistors, and theeleventh, the twelfth, the fifteenth and the sixteenth transistors arePMOS transistors.
 15. The current-mode amplifier as claimed in claim 12,wherein second ends of the ninth, the tenth, the thirteenth and thefourteenth transistors are connected to the first reference voltage, andsecond ends of the eleventh, the twelfth, the fifteenth and thesixteenth transistors are connected to a second reference voltage. 16.The current-mode amplifier as claimed in claim 1, wherein the inputterminal of the output stage comprises a first input terminal and asecond input terminal, the output terminal of the output stage comprisesa first output terminal and a second output terminal, and the outputstage comprises: a ninth transistor, having a first end connected to thefirst output terminal of the output stage, and a control end receiving afirst bias; a tenth transistor, having a first end connected to thesecond output terminal of the output stage, and a control end receivingthe first bias; an eleventh transistor, having a control end connectedto the first input terminal of the output stage; a twelfth transistor,having a control end connected to the second input terminal of theoutput stage; a seventeenth transistor, having a first end connected tothe first end of the ninth transistor, a second end connected to a firstend of the eleventh transistor, and a control end receiving a secondbias; and an eighteenth transistor, having a first end connected to thefirst end of the tenth transistor, a second end connected to a first endof the twelfth transistor, and a control end receiving the second bias.17. The current-mode amplifier as claimed in claim 16, wherein theoutput stage further comprises: a thirteenth transistor, having a firstend connected to the first output terminal of the output stage; afourteenth transistor, having a first end connected to the second outputterminal of the output stage; a first switch, selecting to transmit thefirst bias or a first reference voltage to control ends of thethirteenth transistor and the fourteenth transistor; a fifteenthtransistor, having a control end connected to the first input terminalof the output stage; a sixteenth transistor, having a control endconnected to the second input terminal of the output stage; a nineteenthtransistor, having a first end connected to the first end of thethirteenth transistor, and a second end connected to a first end of thefifteenth transistor; a twentieth transistor, having a first endconnected to the first end of the fourteenth transistor, and a secondend connected to a first end of the sixteenth transistor; and a secondswitch, selecting to transmit the second bias or a second referencevoltage to control ends of the nineteenth transistor and the twentiethtransistor.
 18. The current-mode amplifier as claimed in claim 17,wherein the ninth, the tenth, the thirteenth and the fourteenthtransistors are PMOS transistors, and the eleventh, the twelfth, thefifteenth, the sixteenth, the seventeenth, the eighteenth, thenineteenth and the twentieth transistors are NMOS transistors.
 19. Thecurrent-mode amplifier as claimed in claim 17, wherein the ninth, thetenth, the thirteenth and the fourteenth transistors are NMOStransistors, and the eleventh, the twelfth, the fifteenth, thesixteenth, the seventeenth, the eighteenth, the nineteenth and thetwentieth transistors are PMOS transistors.
 20. The current-modeamplifier as claimed in claim 17, wherein second ends of the ninth, thetenth, the thirteenth and the fourteenth transistors are connected tothe first reference voltage, and second ends of the eleventh, thetwelfth, the fifteenth and the sixteenth transistors are connected tothe second reference voltage.